It is known that useful rubber can be recovered from the guayule shrub Parthenium argentatum gray which grows in semiarid regions of North America, such as the southwest United States and northern Mexico. There has been a renewed interest in the commercialization of guayule rubber to supplement natural rubber supplies obtained from the well-known rubber tree Hevea brazilienses. Both guayule and Hevea are considered "natural" rubber.
Unlike Hevea, guayule rubber lacks substantial amounts of natural anti-oxidants and therefore is usually stabilized with adjuvants to prevent rapid degradation upon contact with air (Guayule, An Alternative Source of Natural Rubber; National Academy of Science; Washington, D.C.; 1977; page 25). Such anti-oxidants or stabilizers for guayule rubber are usually added during the solution phase purification of the crude guayule rubber (ibid, p 36). Since guayule and Hevea rubbers have chemical and physical properties that are virtually identical (ibid, p 3), it is reasonable to expect that conventional anti-oxidents for Hevea rubber (chemically, cis-1,4-polyisoprene) would be adequate for the stabilization of guayule rubber. Surprisingly, however, Keller, Winkler and Stevens (Paper No. 55, "Degradative Effects of Guayule Resin or Natural Rubber," presented at the 117th Meeting of the Rubber Division, ACS; Las Vegas, Nevada, May 1980) found that linoleic acid, which is a component of guayule resin (which in turn is a potential containment of guayule rubber) has an adverse effect on the stability of guayule rubber. Therefore, from a practical standpoint, stabilizer (anti-oxidant) additive(s) for guayule rubber preferably will prevent the deleterious effects of heat and oxygen (air) on the rubber itself (cis-1,4-structure) and at the same time prevents the deleterious effects of guayule resin which may be present in the guayule rubber.
Many chemical compositions have been shown to be effective anti-oxidants for rubber. For example, Dunn in his article, "Review of Unsolved Problems in the Protection of Rubber Against Oxidative Degradation," Rubber Chemistry and Technology 47 960 (1974), stated that over three thousand patents issued in a ten-year period relating to the protection of polymers against aging effects. Dunn also pointed out that because of "antagonism" phenomena, an anti-oxidant which proves highly effective in one rubber may be totally ineffective in another. Howard (Rubber Chemistry and Technology 47, 976 (1974) discussed synergism between various anti-oxidant types.
The following publications disclose the use of specific compounds as stabilizers for guayule rubber:
1. Gonzales (Paper No. 2, 116th Meeting of the Rubber Division, ACS, Cleveland, Ohio, Oct. 23-26, 1979) discloses the results of studies of stabilization of guayule rubber with phenolic compounds (thiomethylene-coupled as well as alkylated) and N,N'-diphenyl-p-phenylenediamine. The synergistic combinations of the present invention were apparently not investigated by Gonzales. PA0 2. Butyl Zimate.RTM., a product of R. T. Vanderbilt Co., Inc. and chemically identified as the zinc salt of di-n-butyldithiocarbamic acid, has been reported to be an excellent stabilizer for guayule rubber (see, for example, Chemical & Engineering News, May 11, 1981). PA0 3. The Chemical and Engineering News disclosure also stated that Butyl Zimate.RTM. alone or in combination with Agerite.RTM. Geltrol.RTM. is the optimum stabilizer for Kraton.RTM. 1107-based hot melts. Agerite.RTM. Geltrol.RTM., another product of R. T. Vanderbilt, is identified chemically in Rubber World; Materials and Compounding Ingredients for Rubber, 1968 Edition, p. 97 as a modified high molecular weight hindered phenol. The same material has also been identified as an alkylated-arylated bisphenolic phosphite in the Vanderbilt Rubber Handbook (page 388), R. T. Vanderbilt Co., Inc. Norwalk, CT, USA 06855 (1978 Edition). PA0 4. Baldemar Motomochi, Posguale G. Galioto and Hans R. Strop disclosed in their publication entitled, "Mechanical Drying of Guayule and Hevea: A Pilot Plant Study" (a paper presented at the Third International Guayule Conference, Pasadena, Calif., Apr. 30, 1980) the use of 50 percent Butyl Zimate.RTM./50 percent AgeRite Superlite.RTM. at a 1 percent level in guayule rubber as a stabilizer system. AgeRite Superlite.RTM. is identified as a mixture of polybutylated Bisphenol A; the latter is, of course, a phenolic compound. No mention, however, of any synergistic interactions was made. PA0 5. "BHT," butylated hydroxy toluene, was reportedly used as an anti-oxidant for stabilization of guayule rubber in the Saltillo, Mexico pilot plant (Paper No. 19, "Vulcanization of Guayule Rubber;" L. F. Ramos and B. Motomochi; presented at the 116th Meeting of the Rubber Division, ACS, Oct. 23-26, 1978 in Cleveland, Ohio).
Use of the synergistic antioxidant combinations of the present invention in guayule rubber has not been reported.